I have the skill “Age of Undeath”. What I normally do is summon 5 skeletal mages and 8 skeletons, cast the AA, unlearn the skeleton skill (so that I have nothing summoned but the AA), then summon 5 skeletal mages and go and fight. The AA does NOT decay health in this scenario, so it is working as the tooltip says.
The issue arises when I equip 2 Hollow Fingers while summoning my skeletons so that I have 10 skeletons instead of 8, for a total of 15. I do the identical process with unlearning the skills then summoning the 5 skeletal mages, but now, the AA decays health just standing there doing nothing at all, with only 5 skeletal mages visible.